1. Technical Field
This invention relates to a system for converting wave, tidal or current energy in a body of flowing water into a more useful form of energy. In particular this invention provides an apparatus for generating electricity using flows such as waves, tides or currents in a body of water.
2. Description of the Related Art
They have been a number of proposals for converting current, wave or tidal flows into electricity as a non-polluting approach to power generation.
Previous systems for extracting energy from waves used the vertical movement of the water surface caused by the phase shift between the elliptical water particle paths along a wavelength. Typically, previous devices involved some form of float coupled to a mechanical arrangement or some form of trapped air body above the wave surface for converting the periodic vertical movement of the wave surface into some form of motion useful for electricity generation (usually rotary). Such systems are often mechanically complicated and to work effectively are tuned to resonate at a frequency at which the energy density of the ambient wave spectrum is expected to peak. Output can drop dramatically if the wave frequency differs from this design resonant frequency. Such systems are useless if there is only lateral flow (current or tide) with no oscillating vertical wave component.
Systems have been proposed for extracting energy in lateral flows. Such systems have involved the use of a vane which can be caused to oscillate by the flow, a mechanical transmission system converting this into rotary motion. These systems face similar problems to wave-powered systems: mechanical complexity, inability to extract energy from other types of motion, etc. Other systems feature a large underwater propeller with an electrical power generator in the hub, analogous to a windmill but for water instead of wind flows. For the swept disc to gain exposure to the maximum incident current energy, the blades have to be very long which in turn requires sophisticated design and materials to accommodate the stresses at the blade root. Offshore tidal barrages seek to concentrate the incident energy of a large cross-section of water flow by trapping the tidal inflow on a rising tide behind a containing wall and funneling it out again through turbines of much smaller cross-sectional area, as in a conventional dam. The falling tide provides an adequate head difference between the trapped water in the lagoon behind the barrage and the falling tide on the seaward side of the barrage. Such barrages, typically across a tidal estuary, are very expensive and environmentally disruptive and require the interaction of the tidal flow with the local topography of the estuary to amplify the tidal range.
One common problem for wave or flow systems is their ability to address a sufficiently large cross-section of the ocean for power generation to be possible on an industrial scale. Furthermore, end or edge effects can make it easier for the flow to go around any structure positioned in the flow to extract energy from it rather than to pass through the energy extraction system. This problem can be lessened by making an installation very large but this in turn can lead to further complexity and expense and may lead beyond the limits of current engineering capability.
WO2008015047 discloses apparatus for converting energy from wave or current flows wherein a series of pipes are arranged such that venturis are defined between the pipes. Water flow between these pipes causes the venturis to act as pumps drawing water through the pipes which are fed by a manifolding flow conduit and driving a turbine. The series of pipes are arranged to form arrays with vertical planes which in turn are mounted on the sea bed to form barrages.